Genome-guided purification of high amounts of the siderophore ornibactin and detection of potentially novel burkholdine derivatives produced by Burkholderia catarinensis 89T.

AIM: The increased availability of genome sequences has enabled the development of valuable tools for the prediction and identification of bacterial natural products. Burkholderia catarinensis 89T produces siderophores and an unknown potent antifungal metabolite. The aim of this work was to identify and purify natural products of B. catarinensis 89T through a genome-guided approach. MATERIALS AND METHODS: The analysis of B. catarinensis 89T genome revealed 16 clusters putatively related to secondary metabolism and antibiotics production. Of particular note was the identification of a nonribosomal peptide synthetase (NRPS) cluster related to the production of the siderophore ornibactin, a hybrid NRPS-polyketide synthase Type 1 cluster for the production of the antifungal glycolipopeptide burkholdine, and a gene cluster encoding homoserine lactones (HSL), probably involved in the regulation of both metabolites. We were able to purify high amounts of the ornibactin derivatives D/C6 and F/C8, while also detecting the derivative B/C4 in mass spectrometry investigations. A group of metabolites with molecular masses ranging from 1188 to 1272 Da could be detected in MS experiments, which we postulate to be new burkholdine analogs produced by B. catarinensis. The comparison of B. catarinensis BGCs with other Bcc members corroborates the hypothesis that this bacterium could produce new derivatives of these metabolites. Moreover, the quorum sensing metabolites C6-HSL, C8-HSL, and 3OH-C8-HSL were observed in LC-MS/MS analysis. CONCLUSION: The new species B. catarinensis is a potential source of new bioactive secondary metabolites. Our results highlight the importance of genome-guided purification and identification of metabolites of biotechnological importance.

The Hydrophobicity and Antifungal Potentiation of Burkholdine Analogues.

The burkholdines are a family of cyclic lipopeptides reported to exhibit antifungal activity. We synthesized a series of 18 burkholdine analogues in good yield by conventional Fmoc-SPPS followed by cyclization with DIPCI/HOBt in the solution phase. Although none of the synthesized peptides exhibited antifungal activity, several did potentiate the antibiotic effect of the antibiotic G418, including the Thr-bearing Bk analogue (4b) and the tartaramide-bearing Bk analogue (5b). This work exemplifies the potential of burkholdine analogues as potentiating agents.

Burkholderia from Fungus Gardens of Fungus-Growing Ants Produces Antifungals That Inhibit the Specialized Parasite Escovopsis.

Within animal-associated microbiomes, the functional roles of specific microbial taxa are often uncharacterized. Here, we use the fungus-growing ant system, a model for microbial symbiosis, to determine the potential defensive roles of key bacterial taxa present in the ants' fungus gardens. Fungus gardens serve as an external digestive system for the ants, with mutualistic fungi in the genus Leucoagaricus converting the plant substrate into energy for the ants. The fungus garden is host to specialized parasitic fungi in the genus Escovopsis. Here, we examine the potential role of Burkholderia spp. that occur within ant fungus gardens in inhibiting Escovopsis. We isolated members of the bacterial genera Burkholderia and Paraburkholderia from 50% of the 52 colonies sampled, indicating that members of the family Burkholderiaceae are common inhabitants in the fungus gardens of a diverse range of fungus-growing ant genera. Using antimicrobial inhibition bioassays, we found that 28 out of 32 isolates inhibited at least one Escovopsis strain with a zone of inhibition greater than 1 cm. Genomic assessment of fungus garden-associated Burkholderiaceae indicated that isolates with strong inhibition all belonged to the genus Burkholderia and contained biosynthetic gene clusters that encoded the production of two antifungals: burkholdine1213 and pyrrolnitrin. Organic extracts of cultured isolates confirmed that these compounds are responsible for antifungal activities that inhibit Escovopsis but, at equivalent concentrations, not Leucoagaricus spp. Overall, these new findings, combined with previous evidence, suggest that members of the fungus garden microbiome play an important role in maintaining the health and function of fungus-growing ant colonies. IMPORTANCE Many organisms partner with microbes to defend themselves against parasites and pathogens. Fungus-growing ants must protect Leucoagaricus spp., the fungal mutualist that provides sustenance for the ants, from a specialized fungal parasite, Escovopsis. The ants take multiple approaches, including weeding their fungus gardens to remove Escovopsis spores, as well as harboring Pseudonocardia spp., bacteria that produce antifungals that inhibit Escovopsis. In addition, a genus of bacteria commonly found in fungus gardens, Burkholderia, is known to produce secondary metabolites that inhibit Escovopsis spp. In this study, we isolated Burkholderia spp. from fungus-growing ants, assessed the isolates' ability to inhibit Escovopsis spp., and identified two compounds responsible for inhibition. Our findings suggest that Burkholderia spp. are often found in fungus gardens, adding another possible mechanism within the fungus-growing ant system to suppress the growth of the specialized parasite Escovopsis.

Synthesis of a burkholdine analogue containing ß-hydroxytyrosine.

Synthesis of a ß-OHTyr-containing Bk analogue, a cyclic octalipopeptide with antifungal activities, is described. Since ß-OHTyr-containing peptides generally are unstable in strong acidic conditions, synthesis of ß-HOTyr-containing peptides by SPPS have rarely been reported. To overcome this problem, we found that using distilled TFA removed the protecting groups of side chains of ß-OHTyr-containing Bk analogue, which was prepared by Fmoc-SPPS. Abbreviations: ß-OHTyr: ß-hydroxytyrosine; ß-OHAsn: ß-hydroxyasparagine; Bk: burkholdine; FAA: fatty acyl amino acid; ß-MeOTyr: ß-methoxytyrosine; SPPS: solid phase peptide synthesis; MIC: minimun inhibitory concentration; DMF: dimethyl formamide; DIPEA: diisopropylethylamine; DIPC: diisopropylcarbodiimide; HOBt: 1-hydroxybenzotriazole; Fmoc: 9-fluorenylmethyloxycarbonyl; HFIP: 1,1,1,3,3,3-hexafluoropropan-2-ol; TFA: trifluoroacetic acid; LAP: N-lauryl -3-amino-4-carbamolypropanoic acid; HPLC: high performance liquid chromatography; ESI-TOFMS: electrospray ionization-time of flight mass spectrometry; Bn: benzyl; Boc: t-butyloxycatbonyl; 2-CTC: 2-chlorotritylchloride.

Structure activity relationship study of burkholdine analogues toward simple antifungal agents.

Cyclic and linear lipopeptides, burkholdine analogues, were synthesized by conventional Fmoc-SPPS and cyclisation with DIPC/HOBt in the solution phase. Synthesized peptides were evaluated for antifungal activities with MIC values against Saccharomyces cerevisiae and Aspergillus oryzae. As a result, the stereochemistry of the amino acid residues and sequences of burkholdine analogues exerted a significant influence on antifungal activities. In addition, we found a linear burkholdine analogue with moderate antifungal activities.